Catalytic dehydrogenation can be used to convert paraffins to the corresponding olefin, e.g., propane to propene, or butane to butene.
FIG. 1 shows one typical arrangement for a moving bed dehydrogenation process 5. The process 5 includes a reactor section 10, a regeneration section 15, and a product recovery section 20.
The reactor section 10 includes one or more reactors 25 (four as shown). The feed 30 is sent to a heat exchanger 35 where it exchanges heat with the reactor effluent 40 to raise the feed temperature. The feed 30 is sent to a preheater 45 where it is heated to the desired inlet temperature. The preheated feed 50 is sent from the preheater 45 to the first reactor 25. Because the dehydrogenation reaction is endothermic, the temperature of the effluent 55 from the first reactor 25 is less than the temperature of the preheated feed 50. The effluent 55 is sent to interstage heaters 60 to raise the temperature to the desired inlet temperature for the next reactor 25.
After the last reactor (in this case the fourth reactor), the effluent 40 is sent to heat exchanger 35, and heat is exchanged with the feed 30. The effluent 40 is then sent to the product recovery section 20.
The catalyst 65 moves through the series of reactors 25. When the catalyst 70 leaves the last reactor 25, it is sent to the regeneration section 15. The regeneration section includes a reactor 75 where the coke on the catalyst is burned off and the catalyst may go through a reconditioning step. The regenerated catalyst 80 is sent back to the first reactor 25.
In the product recovery section 20, the effluent 40 is cooled, compressed, dried, and separated in separator 85. The gas 90 is expanded in expander 95 and then separated into a recycle hydrogen stream 100 and a net separator gas stream 105. The liquid stream 110, which includes the olefin product and unconverted paraffin, is sent for further processing, where the desired olefin product is recovered and the unconverted paraffin is recycled to the dehydrogenation reactor.
FIG. 2 shows a typical arrangement for a cyclic bed dehydrogenation process 115. The process 115 includes a reactor section 120, and a product recovery section similar to that described above (not shown in FIG. 2).
In this process 115, the feed 130 is sent to a heat exchanger 135 where it exchanges heat with the reactor effluent 140 to raise the feed temperature. As shown, there are four reactors 145A-D. Of these, typically one will be operating (145A); one will be purging (145B); one will be regenerating the catalyst, that is, burning of coke and reconditioning if required (145C); and one will be purging and preparing for the next process cycle (145D). The feed 130 is sent to preheaters 150 where it is heated to the desired inlet temperature. The preheated feed 155 is sent from the preheater 150 to the operating reactor 145A.
The effluent 140 from the operating reactor 145A is sent to heat exchanger 135, and heat is exchanged with the feed 130. The effluent 140 is then sent to the product recovery section.
Reactor 145B is being purged. The hydrocarbon feed to the reactor is stopped, and the connection to the effluent is closed. A purge gas 160 is introduced into reactor 145B to remove any hydrocarbon feed from the reactor in preparation for regenerating the catalyst.
Reactor 145C is being regenerated. An oxygen-containing stream 165 is introduced into the reactor so the coke on the catalyst can be burned off, and the catalyst is reconditioned if required.
Reactor 145D is being purged. The oxygen-containing feed to the reactor is stopped. A purge gas 160 is introduced into reactor 145D to remove any residual air/oxygen feed from the reactor in preparation for next processing cycle.
The time duration of steps two, three and four, that is purging, coke burning, and purging is matched with the time duration of the first step, that is the process cycle. In some instances, to match this timing duration, one may use more than one reactor in the processing step.
In paraffin dehydrogenation processes, maximum conversion is limited by equilibrium at the reactor outlet conditions. Feed has to be heated to a high temperature before being fed to a series of adiabatic reactors where dehydrogenation takes place. Depending on the carbon number of the feed being dehydrogenated, this temperature can very from about 450° C. to about 700° C. The lower carbon number feeds, such as ethane, propane, butane (C2-C4), require higher temperatures, in the range of about 600 to about 700° C., compared to those with carbon number, such as decane or dodecane (C10, C12), which may require temperatures in the range of about 450 to about 550° C. As shown in FIG. 3, at 101 kPa (1 atm) and 550° C., the propylene to propane ratio is 32/68, while at the same temperature, the isobutene to isobutane ratio is 50/50.
The paraffin dehydrogenation reaction is equilibrium limited.CnH2n+2CnH2n+H2 
As shown, the dehydrogenation reaction produces alkenes and hydrogen. Because the reaction is endothermic, the reactor outlet temperature is lower than the inlet temperature. As the temperature declines, so does the equilibrium concentration for alkene, and hence it limits the maximum conversion that can be achieved within each reactor. Furthermore, higher inlet temperature can thermally crack the feed hydrocarbons, resulting in selectivity loss.
Multi-stage heating steps increases the circuit pressure drop and hot residence time much more than the required amount for actual hydrocarbon-catalyst contact, resulting in higher utilities consumption and more undesired thermal reactions. Limited conversion increases the amount of recycled unreacted material, resulting in increases in unit capital costs and operating costs.
There is a need for improved dehydrogenation processes.